1. Technical Field
The invention relates to an improved pin for locating a core of a mold used in the investment casting or lost wax process. More particularly, the invention relates to such a pin having a wire center of one metal which is coated with another metal. Specifically, the invention relates to such a pin which resists oxidation during firing of the mold, supports the core of the mold during high-temperature casting and is metallurgically compatible with the casting metal or alloy.
2. Background Information
The investment casting process, or lost wax process, is used to produce hollow cast parts. The mold used to create these hollow parts involves the use of a ceramic core which must be supported by the investment casting pins to hold it in proper position as the remainder of the mold is formed and the final part is cast. The ceramic core is fixed within a wax pattern which is essentially in the form of the final part. The wax pattern is then encased by dipping the core and wax pattern in a ceramic slurry. After the slurry dries, the wax is melted out. The entire assembly is then fired at temperatures typically ranging from 1300 to 1900 degrees Fahrenheit, leaving the hardened ceramic core and shell with casting pins extending therebetween to form the mold for the final part. Thus, the core achieves a proper position within the shell with the aid of the casting pins. Molten metal is poured into the mold to form the cast metal part.
Most often, the mold is fired in an oxidizing environment, although a reducing atmosphere is also possible. Because it is most common to fire in an oxidizing environment, casting pins undergoing such a firing must be resistant to oxidation at these high temperatures. In addition, the pins must be of appropriate material so that no chemical interaction occurs between the pins and the ceramic shell or ceramic core. Casting of metal into the mold typically occurs in a relatively low-oxygen environment and so the concern of oxidizing the pins during casting is reduced. However, there may still be some concern of pin oxidation during casting depending on the specific environment.
During the casting process, it is important that the core of the mold does not shift within the shell. Otherwise, the final part will have walls which are too thick or too thin for the ultimate application. The aerospace and power generation industries, for example, require high-quality parts which must meet close tolerances to provide peak performance and, in many cases, prevent catastrophic failure in an aircraft or power generator. If the core shifts sufficiently so that the final part does not meet such tolerances, the part must be rejected. In order to ensure that the core does not shift or that it shifts only within acceptable tolerances, it is important that the casting pins be sufficiently strong at casting temperatures to sufficiently support the ceramic core. For directionally solidified or single crystal processes, the casting temperature may approach 3,000 degrees Fahrenheit, which limits the possible composition of casting pins for such applications. In addition, to produce high-quality parts free of unacceptable inclusions, chemical reactions or voids which could negatively affect the strength of the final part, the casting pins must be compatible with the metal or alloy of the final part. The composition of the pin must also be chosen so that the pin completely dissolves into the final part, which is typically an alloy. Final parts are commonly formed of special nickel alloys.
Solid platinum casting pins have been used in high temperature castings. However, platinum has become very costly and also suffers from softening or sagging at casting temperatures, thus providing insufficient support for heavier ceramic cores. Therefore, other casting pins are needed which address the above-noted problems in the art.